Method and apparatus for high rate packet data transmission

ABSTRACT

In a data communication system capable of variable rate transmission, high rate packet data transmission improves utilization of the forward link and decreases the transmission delay. Data transmission on the forward link is time multiplexed and the base station transmits at the highest data rate supported by the forward link at each time slot to one mobile station. The data rate is determined by the largest C/I measurement of the forward link signals as measured at the mobile station. Upon determination of a data packet received in error, the mobile station transmits a NACK message back to the base station. The NACK message results in retransmission of the data packet received in error. The data packets can be transmitted out of sequence by the use of sequence number to identify each data unit within the data packets.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a Continuation and claims priorityto co-pending patent application Ser. No. 10/809,213, entitled “METHODAND APPARATUS FOR BURST PILOT FOR A TIME DIVISION MULTIPLEX SYSTEM,”filed Mar. 25, 2004, which is a continuation of U.S. Pat. No. 7,079,550,entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,”issued on Jul. 18, 2006, which is a continuation of U.S. Pat. No.6,574,211, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATATRANSMISSION,” issued on Jun. 3, 2003, and assigned to the assigneehereof and hereby expressly incorporated by reference herein.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to data communication. More particularly,the present invention relates to a novel and improved method andapparatus for high rate packet data transmission.

II. Description of the Related Art

A modern day communication system is required to support a variety ofapplications. One such communication system is a code division multipleaccess (CDMA) system which conforms to the “TIA/EIA/IS-95 MobileStation-Base Station Compatibility Standard for Dual-Mode WidebandSpread Spectrum Cellular System,” hereinafter referred to as the IS-95standard. The CDMA system allows for voice and data communicationsbetween users over a terrestrial link. The use of CDMA techniques in amultiple access communication system is disclosed in U.S. Pat. No.4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATIONSYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No.5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN ACDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of thepresent invention and incorporated by reference herein.

In this specification, base station refers to the hardware with whichthe mobile stations communicate. Cell refers to the hardware or thegeographic coverage area, depending on the context in which the term isused. A sector is a partition of a cell.

Because a sector of a CDMA system has the attributes of a cell, theteachings described in terms of cells are readily extended to sectors.

In the CDMA system, communications between users are conducted throughone or more base stations. A first user on one mobile stationcommunicates to a second user on a second mobile station by transmittingdata on the reverse link to a base station.

The base station receives the data and can route the data to anotherbase station. The data is transmitted on the forward link of the samebase station, or a second base station, to the second mobile station.The forward link refers to transmission from the base station to amobile station and the reverse link refers to transmission from themobile station to a base station. In IS-95 systems, the forward link andthe reverse link are allocated separate frequencies.

The mobile station communicates with at least one base station during acommunication. CDMA mobile stations are capable of communicating withmultiple base stations simultaneously during soft handoff. Soft handoffis the process of establishing a link with a new base station beforebreaking the link with the previous base station. Soft handoff minimizesthe probability of dropped calls. The method and system for providing acommunication with a mobile station through more than one base stationduring the soft handoff process are disclosed in U.S. Pat. No.5,267,261, entitled “MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMACELLULAR COMMUNICATIONS SYSTEM,” assigned to the assignee of the presentinvention and incorporated by reference herein. Softer handoff is theprocess whereby the communication occurs over multiple sectors which areserviced by the same base station. The process of softer handoff isdescribed in detail in U.S. patent application Ser. No. 08/763,498,entitled “METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OFA COMMON BASE STATION,” filed Dec. 11, 1996, now U.S. Pat. No.5,933,787, issued Aug. 3, 1999, by Klein S. Gilhousen et al., assignedto the assignee of the present invention and incorporated by referenceherein.

Given the growing demand for wireless data applications, the need forvery efficient wireless data communication systems has becomeincreasingly significant. The IS-95 standard is capable of transmittingtraffic data and voice data over the forward and reverse links. A methodfor transmitting traffic data in code channel frames of fixed size isdescribed in detail in U.S. Pat. No. 5,504,773, entitled “METHOD ANDAPPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,” assigned to theassignee of the present invention and incorporated by reference herein.

In accordance with the IS-95 standard, the traffic data or voice data ispartitioned into code channel frames which are 20 msec. wide with datarates as high as 14.4 Kbps.

A significant difference between voice services and data services is thefact that the former imposes stringent and fixed delay requirements.Typically, the overall one-way delay of speech frames must be less than100 msec. In contrast, the data delay can become a variable parameterused to optimize the efficiency of the data communication system.Specifically, more efficient error correcting coding techniques whichrequire significantly larger delays than those that can be tolerated byvoice services can be utilized. An exemplary efficient coding scheme fordata is disclosed in U.S. patent application Ser. No. 08/743,688,entitled “SOFT DECISION OUTPUT DECODER FOR DECODING CONVOLUTIONALLYENCODED CODEWORDS,” filed Nov. 6, 1996, now U.S. Pat. No. 5,933,462,issued Aug. 3, 1999, by Andrew J. Viterbi et al., assigned to theassignee of the present invention and incorporated by reference herein.

Another significant difference between voice services and data servicesis that the former requires a fixed and common grade of service (GOS)for all users. Typically, for digital systems providing voice services,this translates into a fixed and equal transmission rate for all usersand a maximum tolerable value for the error rates of the speech frames.In contrast, for data services, the GOS can be different from user touser and can be a parameter optimized to increase the overall efficiencyof the data communication system. The GOS of a data communication systemis typically defined as the total delay incurred in the transfer of apredetermined amount of data, hereinafter referred to as a data packet.

Yet another significant difference between voice services and dataservices is that the former requires a reliable communication linkwhich, in the exemplary CDMA communication system, is provided by softhandoff. Soft handoff results in redundant transmissions from two ormore base stations to improve reliability. However, this additionalreliability is not required for data transmission because the datapackets received in error can be retransmitted. For data services, thetransmit power used to support soft handoff can be more efficiently usedfor transmitting additional data.

The parameters which measure the quality and effectiveness of a datacommunication system are the transmission delay required to transfer adata packet and the average throughput rate of the system. Transmissiondelay does not have the same impact in data communication as it does forvoice communication, but it is an important metric for measuring thequality of the data communication system. The average throughput rate isa measure of the efficiency of the data transmission capability of thecommunication system.

It is well known that in cellular systems the signal-to-noise andinterference ratio (C/I) of any given user is a function of the locationof the user within the coverage area.

In order to maintain a given level of service, TDMA and FDMA systemsresort to frequency reuse techniques, i.e., not all frequency channelsand/or time slots are used in each base station. In a CDMA system, thesame frequency allocation is reused in every cell of the system, therebyimproving the overall efficiency. The C/I that any given user's mobilestation achieves determines the information rate that can be supportedfor this particular link from the base station to the user's mobilestation. Given the specific modulation and error correction method usedfor the transmission, which the present invention seek to optimize fordata transmissions, a given level of performance is achieved at acorresponding level of C/I. For idealized cellular system with hexagonalcell layouts and utilizing a common frequency in every cell, thedistribution of C/I achieved within the idealized cells can becalculated.

The C/I achieved by any given user is a function of the path loss, whichfor terrestrial cellular systems increases as r³ to r⁵, where r is thedistance to the radiating source. Furthermore, the path loss is subjectto random variations due to man-made or natural obstructions within thepath of the radio wave. These random variations are typically modeled asa lognormal shadowing random process with a standard deviation of 8 dB.The resulting C/I distribution achieved for an ideal hexagonal cellularlayout with omni-directional base station antennas, r⁴ propagation law,and shadowing process with 8 dB standard deviation is shown in FIG. 10.

The obtained C/I distribution can only be achieved if, at any instant intime and at any location, the mobile station is served by the best basestation which is defined as that achieving the largest C/I value,regardless of the physical distance to each base station. Because of therandom nature of the path loss as described above, the signal with thelargest C/I value can be one, which is other than the minimum physicaldistance from the mobile station. In contrast, if a mobile station wasto communicate only via the base station of minimum distance, the C/Ican be substantially degraded. It is therefore beneficial for mobilestations to communicate to and from the best serving base station at alltimes, thereby achieving the optimum C/I value. It can also be observedthat the range of values of the achieved C/l, in the above idealizedmodel and as shown in FIG. 10, is such that the difference between thehighest and lowest value can be as large as 10,000. In practicalimplementation the range is typically limited to approximately 1:100 or20 dB. It is therefore possible for a CDMA base station to serve mobilestations with information bit rates that can vary by as much as a factorof 100, since the following relationship holds: $\begin{matrix}{{R_{b} = {W\frac{\left( {C/I} \right)}{\left( {E_{b}/I_{o}} \right)}}},} & (1)\end{matrix}$where R_(b) represents the information rate to a particular mobilestation, W is the total bandwidth occupied by the spread spectrumsignal, and E_(b)/I_(o) is the energy per bit over interference densityrequired to achieve a given level of performance. For instance, if thespread spectrum signal occupies a bandwidth W of 1.2288 MHz and reliablecommunication requires an average E_(b)/I_(o) equal to 3 dB, then amobile station which achieves a C/I value of 3 dB to the best basestation can communicate at a data rate as high as 1.2288 Mbps. On theother hand, if a mobile station is subject to substantial interferencefrom adjacent base stations and can only achieve a C/I of −7 dB,reliable communication cannot be supported at a rate greater than 122.88Kbps. A communication system designed to optimize the average throughputwill therefore attempts to serve each remote user from the best servingbase station and at the highest data rate R_(b) which the remote usercan reliably support. The data communication system of the presentinvention exploits the characteristic cited above and optimizes the datathroughput from the CDMA base stations to the mobile stations.

SUMMARY

One example provides a receiver method and apparatus for determining amodulation format for transmission as a function of channel quality of alink in a communication system. The receiver periodically transmits aquality indicator which maps to a transmission data rate. In response,the receiver receives data as a function of the quality indicator. Thetransmitter periodically receives a quality indicator, wherein thequality indicator maps to a transmission data rate, and the transmittertransmits data as a function of the quality indicator.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a diagram of a data communication system of the presentinvention comprising a plurality of cells, a plurality of base stationsand a plurality of mobile stations;

FIG. 2 is an exemplary block diagram of the subsystems of the datacommunication system of the present invention;

FIGS. 3A-3B are block diagrams of the exemplary forward linkarchitecture of the present invention;

FIG. 4A is a diagram of the exemplary forward link frame structure ofthe present invention;

FIGS. 4B-4C are diagrams of the exemplary forward traffic channel andpower control channel, respectively;

FIG. 4D is a diagram of the punctured packet of the present invention;

FIGS. 4E-4G are diagrams of the two exemplary data packet formats andthe control channel capsule, respectively;

FIG. 5 is an exemplary timing diagram showing the high rate packettransmission on the forward link;

FIG. 6 is a block diagram of the exemplary reverse link architecture ofthe present invention;

FIG. 7A is a diagram of the exemplary reverse link frame structure ofthe present invention;

FIG. 7B is a diagram of the exemplary reverse link access channel;

FIG. 8 is an exemplary timing diagram showing the high rate datatransmission on the reverse link;

FIG. 9 is an exemplary state diagram showing the transitions between thevarious operating states of the mobile station; and

FIG. 10 is a diagram of the cumulative distribution function (CDF) ofthe C/I distribution in an ideal hexagonal cellular layout.

DETAILED DESCRIPTION

In accordance with the exemplary embodiment of the data communicationsystem of the present invention, forward link data transmission occursfrom one base station to one mobile station (see FIG. 1) at or near themaximum data rate which can be supported by the forward link and thesystem. Reverse link data communication can occur from one mobilestation to one or more base stations. The calculation of the maximumdata rate for forward link transmission is described in detail below.Data is partitioned into data packets, with each data packet beingtransmitted over one or more time slots (or slots). At each time slot,the base station can direct data transmission to any mobile stationwhich is in communication with the base station.

Initially, the mobile station establishes communication with a basestation using a predetermined access procedure. In this connected state,the mobile station can receive data and control messages from the basestation, and is able to transmit data and control messages to the basestation. The mobile station then monitors the forward link fortransmissions from the base stations in the active set of the mobilestation. The active set contains a list of base stations incommunication with the mobile station. Specifically, the mobile stationmeasures the signal-to-noise-and-interference ratio (C/I) of the forwardlink pilot from the base stations in the active set, as received at themobile station. If the received pilot signal is above a predeterminedadd threshold or below a predetermined drop threshold, the mobilestation reports this to the base station. Subsequent messages from thebase station direct the mobile station to add or delete the basestation(s) to or from its active set, respectively. The variousoperating states of the mobile station are described below.

If there is no data to send, the mobile station returns to an idle stateand discontinues transmission of data rate information to the basestation(s). While the mobile station is in the idle state, the mobilestation monitors the control channel from one or more base stations inthe active set for paging messages.

If there is data to be transmitted to the mobile station, the data issent by a central controller to all base stations in the active set andstored in a queue at each base station. A paging message is then sent byone or more base stations to the mobile station on the respectivecontrol channels. The base station may transmit all such paging messagesat the same time across several base stations in order to ensurereception even when the mobile station is switching between basestations. The mobile station demodulates and decodes the signals on oneor more control channels to receive the paging messages.

Upon decoding the paging messages, and for each time slot until the datatransmission is completed, the mobile station measures the C/I of theforward link signals from the base stations in the active set, asreceived at the mobile station. The C/I of the forward link signals canbe obtained by measuring the respective pilot signals. The mobilestation then selects the best base station based on a set of parameters.The set of parameters can comprise the present and previous C/Imeasurements and the bit-error-rate or packet-error-rate. For example,the best base station can be selected based on the largest C/Imeasurement. The mobile station then identifies the best base stationand transmits to the selected base station a data request message(hereinafter referred to as the DRC message) on the data request channel(hereinafter referred to as the DRC channel). The DRC message cancontain the requested data rate or, alternatively, an indication of thequality of the forward link channel (e.g., the C/I measurement itself,the bit-error-rate, or the packet-error-rate). In the exemplaryembodiment, the mobile station can direct the transmission of the DRCmessage to a specific base station by the use of a Walsh code, whichuniquely identifies the base station. The DRC message symbols areexclusively OR'ed (XOR) with the unique Walsh code. Since each basestation in the active set of the mobile station is identified by aunique Walsh code, only the selected base station which performs theidentical XOR operation as that performed by the mobile station, withthe correct Walsh code, can correctly decode the DRC message. The basestation uses the rate control information from each mobile station toefficiently transmit forward link data at the highest possible rate.

At each time slot, the base station can select any of the paged mobilestations for data transmission. The base station then determines thedata rate at which to transmit the data to the selected mobile stationbased on the most recent value of the DRC message received from themobile station. Additionally, the base station uniquely identifies atransmission to a particular mobile station by using a spreading code,which is unique to that mobile station. In the exemplary embodiment,this spreading code is the long pseudo noise (PN) code, which is definedby IS-95 standard.

The mobile station, for which the data packet is intended, receives thedata transmission and decodes the data packet. Each data packetcomprises a plurality of data units. In the exemplary embodiment, a dataunit comprises eight information bits, although different data unitsizes can be defined and are within the scope of the present invention.In the exemplary embodiment, each data unit is associated with asequence number and the mobile stations are able to identify eithermissed or duplicative transmissions. In such events, the mobile stationscommunicate via the reverse link data channel the sequence numbers ofthe missing data units. The base station controllers, which receive thedata messages from the mobile stations, then indicate to all basestations communicating with this particular mobile station which dataunits were not received by the mobile station. The base stations thenschedule a retransmission of such data units. Each mobile station in thedata communication system can communicate with multiple base stations onthe reverse link. In the exemplary embodiment, the data communicationsystem of the present invention supports soft handoff and softer handoffon the reverse link for several reasons. First, soft handoff does notconsume additional capacity on the reverse link but rather allows themobile stations to transmit data at the minimum power level such that atleast one of the base stations can reliably decode the data. Second,reception of the reverse link signals by more base stations increasesthe reliability of the transmission and only requires additionalhardware at the base stations.

In the exemplary embodiment, the forward link capacity of the datatransmission system of the present invention is determined by the raterequests of the mobile stations. Additional gains in the forward linkcapacity can be achieved by using directional antennas and/or adaptivespatial filters. An exemplary method and apparatus for providingdirectional transmissions are disclosed in U.S. patent application Ser.No. 08/575,049, entitled “METHOD AND APPARATUS FOR DETERMINING THETRANSMISSION DATA RATE IN A MULTI-USER COMMUNICATION SYSTEM,” filed Dec.20, 1995, now U.S. Pat. No. 5,857,147, issued Jan. 5, 1999, by WilliamR. Gardner et al., and U.S. patent application Ser. No. 08/925,521,entitled “METHOD AND APPARATUS FOR PROVIDING ORTHOGONAL SPOT BEAMS,SECTORS, AND PICOCELLS,” filed Sep. 8, 1997, now U.S. Pat. No.6,285,655, issued Sep. 4, 2001, by Stein A. Lundby et al., both assignedto the assignee of the present invention and incorporated by referenceherein.

I. System Description

Referring to the figures, FIG. 1 represents the exemplary datacommunication system of the present invention which comprises multiplecells 2 a-2 g. Each cell 2 is serviced by a corresponding base station4. Various mobile stations 6 are dispersed throughout the datacommunication system. In the exemplary embodiment, each of mobilestations 6 communicates with at most one base station 4 on the forwardlink at each time slot but can be in communication with one or more basestations 4 on the reverse link, depending on whether the mobile station6 is in soft handoff. For example, base station 4 a transmits dataexclusively to mobile station 6 a, base station 4 b transmits dataexclusively to mobile station 6 b, and base station 4 c transmits dataexclusively to mobile station 6 c on the forward link at time slot n. InFIG. 1, the solid line with the arrow indicates a data transmission frombase station 4 to mobile station 6. A broken line with the arrowindicates that mobile station 6 is receiving the pilot signal, but nodata transmission, from base station 4. The reverse link communicationis not shown in FIG. 1 for simplicity.

As shown by FIG. 1, each base station 4 preferably transmits data to onemobile station 6 at any given moment. Mobile stations 6, especiallythose located near a cell boundary, can receive the pilot signals frommultiple base stations 4. If the pilot signal is above a predeterminedthreshold, mobile station 6 can request that base station 4 be added tothe active set of mobile station 6. In the exemplary embodiment, mobilestation 6 can receive data transmission from zero or one member of theactive set.

A block diagram illustrating the basic subsystems of the datacommunication system of the present invention is shown in FIG. 2. Basestation controller 10 interfaces with packet network interface 24, PSTN30, and all base stations 4 in the data communication system (only onebase station 4 is shown in FIG. 2 for simplicity). Base stationcontroller 10 coordinates the communication between mobile stations 6 inthe data communication system and other users connected to packetnetwork interface 24 and PSTN 30. PSTN 30 interfaces with users throughthe standard telephone network (not shown in FIG. 2).

Base station controller 10 contains many selector elements 14, althoughonly one is shown in FIG. 2 for simplicity. One selector element 14 isassigned to control the communication between one or more base stations4 and one mobile station 6. If selector element 14 has not been assignedto mobile station 6, call control processor 16 is informed of the needto page mobile station 6. Call control processor 16 then directs basestation 4 to page mobile station 6.

Data source 20 contains the data which is to be transmitted to mobilestation 6. Data source 20 provides the data to packet network interface24. Packet network interface 24 receives the data and routes the data toselector element 14. Selector element 14 sends the data to each basestation 4 in communication with mobile station 6. Each base station 4maintains data queue 40, which contains the data to be transmitted tomobile station 6.

In the exemplary embodiment, on the forward link, a data packet refersto a predetermined amount of data, which is independent of the datarate. The data packet is formatted with other control and coding bitsand encoded. If data transmission occurs over multiple Walsh channels,the encoded packet is demultiplexed into parallel streams, with eachstream transmitted over one Walsh channel.

The data is sent, in data packets, from data queue 40 to channel element42. For each data packet, channel element 42 inserts the necessarycontrol fields. The data packet, control fields, frame check sequencebits, and code tail bits comprise a formatted packet. Channel element 42then encodes one or more formatted packets and interleaves (or reorders)the symbols within the encoded packets. Next, the interleaved packet isscrambled with a scrambling sequence, covered with Walsh covers, andspread with the long PN code and the short PN_(I) and PN_(Q) codes. Thespread data is quadrature modulated, filtered, and amplified by atransmitter within RF unit 44. The forward link signal is transmittedover the air through antenna 46 on forward link 50.

At mobile station 6, the forward link signal is received by antenna 60and routed to a receiver within front end 62. The receiver filters,amplifies, quadrature demodulates, and quantizes the signal. Thedigitized signal is provided to demodulator (DEMOD) 64 where it isdespread with the long PN code and the short PN_(I) and PN_(Q) codes,decovered with the Walsh covers, and descrambled with the identicalscrambling sequence. The demodulated data is provided to decoder 66which performs the inverse of the signal processing functions done atbase station 4, specifically the de-interleaving, decoding, and framecheck functions. The decoded data is provided to data sink 68. Thehardware, as described above, supports transmissions of data, messaging,voice, video, and other communications over the forward link.

The system control and scheduling functions can be accomplished by manyimplementations. The location of channel scheduler 48 is dependent onwhether a centralized or distributed control/scheduling processing isdesired. For example, for distributed processing, channel scheduler 48can be located within each base station 4. Conversely, for centralizedprocessing, channel scheduler 48 can be located within base stationcontroller 10 and can be designed to coordinate the data transmissionsof multiple base stations 4. Other implementations of the abovedescribed functions can be contemplated and are within the scope of thepresent invention.

As shown in FIG. 1, mobile stations 6 are dispersed throughout the datacommunication system and can be in communication with zero or one basestation 4 on the forward link. In the exemplary embodiment, channelscheduler 48 coordinates the forward link data transmissions of one basestation 4. In the exemplary embodiment, channel scheduler 48 connects todata queue 40 and channel element 42 within base station 4 and receivesthe queue size, which is indicative of the amount of data to transmit tomobile station 6, and the DRC messages from mobile stations 6. Channelscheduler 48 schedules high rate data transmission such that the systemgoals of maximum data throughput and minimum transmission delay areoptimized.

In the exemplary embodiment, the data transmission is scheduled based inpart on the quality of the communication link. An exemplarycommunication system which selects the transmission rate based on thelink quality is disclosed in U.S. patent application Ser. No.08/741,320, entitled “METHOD AND APPARATUS FOR PROVIDING HIGH SPEED DATACOMMUNICATIONS IN A CELLULAR ENVIRONMENT,” filed Sep. 11, 1996, now U.S.Pat. No. 6,496,543, issued Dec. 17, 2002, by Ephraim Zehavi, assigned tothe assignee of the present invention and incorporated by referenceherein. In the present invention, the scheduling of the datacommunication can be based on additional considerations such as the GOSof the user, the queue size, the type of data, the amount of delayalready experienced, and the error rate of the data transmission. Theseconsiderations are described in detail in U.S. patent application Ser.No. 08/798,951, entitled “METHOD AND APPARATUS FOR FORWARD LINK RATESCHEDULING,” filed Feb. 11, 1997, now U.S. Pat. No. 6,335,922, issuedJan. 1, 2002, by Edward G. Tiedemann Jr. et al., and U.S. patentapplication Ser. No. 08/835,632, entitled “METHOD AND APPARATUS FORREVERSE LINK RATE SCHEDULING,” filed Aug. 20, 1997, now U.S. Pat. No.5,914,950, issued Jun. 22, 1999, by Tao Chen et al., both are assignedto the assignee of the present invention and incorporated by referenceherein. Other factors can be considered in scheduling data transmissionsand are within the scope of the present invention.

The data communication system of the present invention supports data andmessage transmissions on the reverse link. Within mobile station 6,controller 76 processes the data or message transmission by routing thedata or message to encoder 72. Controller 76 can be implemented in amicrocontroller, a microprocessor, a digital signal processing (DSP)chip, or an ASIC programmed to perform the function as described herein.

In the exemplary embodiment, encoder 72 encodes the message consistentwith the Blank and Burst signaling data format described in theaforementioned U.S. Pat. No. 5,504,773. Encoder 72 then generates andappends a set of CRC bits, appends a set of code tail bits, encodes thedata and appended bits, and reorders the symbols within the encodeddata. The interleaved data is provided to modulator (MOD) 74.

Modulator 74 can be implemented in many embodiments. In the exemplaryembodiment (see FIG. 6), the interleaved data is covered with Walshcodes, spread with a long PN code, and further spread with the short PNcodes. The spread data is provided to a transmitter within front end 62.The transmitter modulates, filters, amplifies, and transmits the reverselink signal over the air, through antenna 60, on reverse link 52.

In the exemplary embodiment, mobile station 6 spreads the reverse linkdata in accordance with a long PN code. Each reverse link channel isdefined in accordance with the temporal offset of a common long PNsequence. At two differing offsets the resulting modulation sequencesare uncorrelated. The offset of a mobile station 6 is determined inaccordance with a unique numerical identification of mobile station 6,which in the exemplary embodiment of the IS-95 mobile stations 6 is themobile station specific identification number. Thus, each mobile station6 transmits on one uncorrelated reverse link channel determined inaccordance with its unique electronic serial number.

At base station 4, the reverse link signal is received by antenna 46 andprovided to RF unit 44. RF unit 44 filters, amplifies, demodulates, andquantizes the signal and provides the digitized signal to channelelement 42. Channel element 42 despreads the digitized signal with theshort PN codes and the long PN code. Channel element 42 also performsthe Walsh code decovering and pilot and DRC extraction. Channel element42 then reorders the demodulated data, decodes the de-interleaved data,and performs the CRC check function. The decoded data, e.g., the data ormessage, is provided to selector element 14. Selector element 14 routesthe data and message to the appropriate destination. Channel element 42may also forward a quality indicator to selector element 14 indicativeof the condition of the received data packet.

In the exemplary embodiment, mobile station 6 can be in one of threeoperating states. An exemplary state diagram showing the transitionsbetween the various operating states of mobile station 6 is shown inFIG. 9. In the access state 902, mobile station 6 sends access probesand waits for channel assignment by base station 4. The channelassignment comprises allocation of resources, such as a power controlchannel and frequency allocation. Mobile station 6 can transition fromthe access state 902 to the connected state 904 if mobile station 6 ispaged and alerted to an upcoming data transmission, or if mobile station6 transmits data on the reverse link. In the connected state 904, mobilestation 6 exchanges (e.g., transmits or receives) data and performshandoff operations. Upon completion of a release procedure, mobilestation 6 transitions from the connected state 904 to the idle state906. Mobile station 6 can also transmission from the access state 902 tothe idle state 906 upon being rejected of a connection with base station4. In the idle state 906, mobile station 6 listens to overhead andpaging messages by receiving and decoding messages on the forwardcontrol channel and performs idle handoff procedure. Mobile station 6can transition to the access state 902 by initiating the procedure. Thestate diagram shown in FIG. 9 is only an exemplary state definition,which is shown for illustration. Other state diagrams can also beutilized and are within the scope of the present invention.

II. Forward Link Data Transmission

In the exemplary embodiment, the initiation of a communication betweenmobile station 6 and base station 4 occurs in a similar manner as thatfor the CDMA system. After completion of the call set up, mobile station6 monitors the control channel for paging messages. While in theconnected state, mobile station 6 begins transmission of the pilotsignal on the reverse link.

An exemplary flow diagram of the forward link high rate datatransmission of the present invention is shown in FIG. 5. If basestation 4 has data to transmit to mobile station 6, base station 4 sendsa paging message addressed to mobile station 6 on the control channel atblock 502. The paging message can be sent from one or multiple basestations 4, depending on the handoff state of mobile station 6. Uponreception of the paging message, mobile station 6 begins the C/Imeasurement process at block 504. The C/I of the forward link signal iscalculated from one or a combination of methods described below. Mobilestation 6 then selects a requested data rate based on the best C/Imeasurement and transmits a DRC message on the DRC channel at block 506.

Within the same time slot, base station 4 receives the DRC message atblock 508. If the next time slot is available for data transmission,base station 4 transmits data to mobile station 6 at the requested datarate at block 510. Mobile station 6 receives the data transmission atblock 512. If the next time slot is available, base station 4 transmitsthe remainder of the packet at block 514 and mobile station 6 receivesthe data transmission at block 516.

In the present invention, mobile station 6 can be in communication withone or more base stations 4 simultaneously. The actions taken by mobilestation 6 depend on whether mobile station 6 is or is not in softhandoff. These two cases are discussed separately below.

III. No Handoff Case

In the no handoff case, mobile station 6 communicates with one basestation 4. Referring to FIG. 2, the data destined for a particularmobile station 6 is provided to selector element 14 which has beenassigned to control the communication with that mobile station 6.Selector element 14 forwards the data to data queue 40 within basestation 4. Base station 4 queues the data and transmits a paging messageon the control channel. Base station 4 then monitors the reverse linkDRC channel for DRC messages from mobile station 6. If no signal isdetected on the DRC channel, base station 4 can retransmit the pagingmessage until the DRC message is detected. After a predetermined numberof retransmission attempts, base station 4 can terminate the process orre-initiate a call with mobile station 6.

In the exemplary embodiment, mobile station 6 transmits the requesteddata rate, in the form of a DRC message, to base station 4 on the DRCchannel. In the alternative embodiment, mobile station 6 transmits anindication of the quality of the forward link channel (e.g., the C/Imeasurement) to base station 4. In the exemplary embodiment, the 3-bitDRC message is decoded with soft decisions by base station 4. In theexemplary embodiment, the DRC message is transmitted within the firsthalf of each time slot. Base station 4 then has the remaining half ofthe time slot to decode the DRC message and configure the hardware fordata transmission at the next successive time slot, if that time slot isavailable for data transmission to this mobile station 6. If the nextsuccessive time slot is not available, base station 4 waits for the nextavailable time slot and continues to monitor the DRC channel for the newDRC messages.

In the first embodiment, base station 4 transmits at the requested datarate. This embodiment confers to mobile station 6 the important decisionof selecting the data rate. Always transmitting at the requested datarate has the advantage that mobile station 6 knows which data rate toexpect. Thus, mobile station 6 only demodulates and decodes the trafficchannel in accordance with the requested data rate. Base station 4 doesnot have to transmit a message to mobile station 6 indicating which datarate is being used by base station 4.

In the first embodiment, after reception of the paging message, mobilestation 6 continuously attempts to demodulate the data at the requesteddata rate. Mobile station 6 demodulates the forward traffic channel andprovides the soft decision symbols to the decoder. The decoder decodesthe symbols and performs the frame check on the decoded packet todetermine whether the packet was received correctly. If the packet wasreceived in error or if the packet was directed for another mobilestation 6, the frame check would indicate a packet error. Alternativelyin the first embodiment, mobile station 6 demodulates the data on aslot-by-slot basis. In the exemplary embodiment, mobile station 6 isable to determine whether a data transmission is directed for it basedon a preamble which is incorporated within each transmitted data packet,as described below. Thus, mobile station 6 can terminate the decodingprocess if it is determined that the transmission is directed foranother mobile station 6. In either case, mobile station 6 transmits anegative acknowledgments (NACK) message to base station 4 to acknowledgethe incorrect reception of the data units. Upon receipt of the NACKmessage, the data units received in error is retransmitted.

The transmission of the NACK messages can be implemented in a mannersimilar to the transmission of the error indicator bit (EIB) in the CDMAsystem. The implementation and use of EIB transmission are disclosed inU.S. Pat. No. 5,568,483, entitled “METHOD AND APPARATUS FOR THEFORMATTING OF DATA FOR TRANSMISSION,” assigned to the assignee of thepresent invention and incorporated by reference herein. Alternatively,NACK can be transmitted with messages.

In the second embodiment, the data rate is determined by base station 4with input from mobile station 6. Mobile station 6 performs the C/Imeasurement and transmits an indication of the link quality (e.g., theC/I measurement) to base station 4. Base station 4 can adjust therequested data rate based on the resources available to base station 4,such as the queue size and the available transmit power. The adjusteddata rate can be transmitted to mobile station 6 prior to or concurrentwith data transmission at the adjusted data rate, or can be implied inthe encoding of the data packets. In the first case, wherein mobilestation 6 receives the adjusted data rate before the data transmission,mobile station 6 demodulates and decodes the received packet in themanner described in the first embodiment. In the second case, whereinthe adjusted data rate is transmitted to mobile station 6 concurrentwith the data transmission, mobile station 6 can demodulate the forwardtraffic channel and store the demodulated data. Upon receipt of theadjusted data rate, mobile station 6 decodes the data in accordance withthe adjusted data rate. And in the third case, wherein the adjusted datarate is implied in the encoded data packets, mobile station 6demodulates and decodes all candidate rates and determine a posteriorithe transmit rate for selection of the decoded data. The method andapparatus for performing rate determination are described in detail inU.S. patent application Ser. No. 08/730,863, entitled “METHOD ANDAPPARATUS FOR DETERMINING THE RATE OF RECEIVED DATA IN A VARIABLE RATECOMMUNICATION SYSTEM,” filed Oct. 18, 1996, now U.S. Pat. No. 5,751,725,issued May 12, 1998, by Tao Chen, and U.S. patent application Ser. No.08/908,866, also entitled “METHOD AND APPARATUS FOR DETERMINING THE RATEOF RECEIVED DATA IN A VARIABLE RATE COMMUNICATION SYSTEM,” filed Aug.17, 1999, now U.S. Pat. No. 6,175,590, issued Jan. 16, 2001, by JeremyM. Stein, both assigned to the assignee of the present invention andincorporated by reference herein. For all cases described above, mobilestation 6 transmits a NACK message as described above if the outcome ofthe frame check is negative.

The discussion hereinafter is based on the first embodiment whereinmobile station 6 transmits to base station 4 the DRC message indicativeof the requested data rate, except as otherwise indicated. However, theinventive concept described herein is equally applicable to the secondembodiment wherein mobile station 6 transmits an indication of the linkquality to base station 4.

IV. Handoff Case

In the handoff case, mobile station 6 communicates with multiple basestations 4 on the reverse link. In the exemplary embodiment, datatransmission on the forward link to a particular mobile station 6 occursfrom one base station 4. However, mobile station 6 can simultaneouslyreceive the pilot signals from multiple base stations 4. If the C/Imeasurement of a base station 4 is above a predetermined threshold, thebase station 4 is added to the active set of mobile station 6. Duringthe soft handoff direction message, the new base station 4 assignsmobile station 6 to a reverse power control (RPC) Walsh channel which isdescribed below. Each base station 4 in soft handoff with mobile station6 monitors the reverse link transmission and sends an RPC bit on theirrespective RPC Walsh channels.

Referring to FIG. 2, selector element 14 assigned to control thecommunication with mobile station 6 forwards the data to all basestations 4 in the active set of mobile station 6. All base stations 4which receive data from selector element 14 transmit a paging message tomobile station 6 on their respective control channels. When mobilestation 6 is in the connected state, mobile station 6 performs twofunctions. First, mobile station 6 selects the best base station 4 basedon a set of parameter which can be the best C/I measurement. Mobilestation 6 then selects a data rate corresponding to the C/I measurementand transmits a DRC message to the selected base station 4. Mobilestation 6 can direct transmission of the DRC message to a particularbase station 4 by covering the DRC message with the Walsh cover assignedto that particular base station 4. Second, mobile station 6 attempts todemodulate the forward link signal in accordance with the requested datarate at each subsequent time slot.

After transmitting the paging messages, all base stations 4 in theactive set monitor the DRC channel for a DRC message from mobile station6. Again, because the DRC message is covered with a Walsh code, theselected base station 4 assigned with the identical Walsh cover is ableto decover the DRC message. Upon receipt of the DRC message, theselected base station 4 transmits data to mobile station 6 at the nextavailable time slots.

In the exemplary embodiment, base station 4 transmits data in packetscomprising a plurality of data units at the requested data rate tomobile station 6. If the data units are incorrectly received by mobilestation 6, a NACK message is transmitted on the reverse links to allbase stations 4 in the active set. In the exemplary embodiment, the NACKmessage is demodulated and decoded by base stations 4 and forwarded toselector element 14 for processing. Upon processing of the NACK message,the data units are retransmitted using the procedure as described above.In the exemplary embodiment, selector element 14 combines the NACKsignals received from all base stations 4 into one NACK message andsends the NACK message to all base stations 4 in the active set.

In the exemplary embodiment, mobile station 6 can detect changes in thebest C/I measurement and dynamically request data transmissions fromdifferent base stations 4 at each time slot to improve efficiency. Inthe exemplary embodiment, since data transmission occurs from only onebase station 4 at any given time slot, other base stations 4 in theactive set may not be aware which data units, if any, has beentransmitted to mobile station 6. In the exemplary embodiment, thetransmitting base station 4 informs selector element 14 of the datatransmission. Selector element 14 then sends a message to all basestations 4 in the active set. In the exemplary embodiment, thetransmitted data is presumed to have been correctly received by mobilestation 6. Therefore, if mobile station 6 requests data transmissionfrom a different base station 4 in the active set, the new base station4 transmits the remaining data units. In the exemplary embodiment, thenew base station 4 transmits in accordance with the last transmissionupdate from selector element 14. Alternatively, the new base station 4selects the next data units to transmit using predictive schemes basedon metrics such as the average transmission rate and prior updates fromselector element 14. These mechanisms minimize duplicativeretransmissions of the same data units by multiple base stations 4 atdifferent time slots which result in a loss in efficiency. If a priortransmission was received in error, base stations 4 can retransmit thosedata units out of sequence since each data unit is identified by aunique sequence number as described below. In the exemplary embodiment,if a hole (or non-transmitted data units) is created (e.g., as theresult of handoff between one base station 4 to another base station 4),the missing data units are considered as though received in error.Mobile station 6 transmits NACK messages corresponding to the missingdata units and these data units are retransmitted.

In the exemplary embodiment, each base station 4 in the active setmaintains an independent data queue 40 which contains the data to betransmitted to mobile station 6. The selected base station 4 transmitsdata existing in its data queue 40 in a sequential order, except forretransmissions of data units received in error and signaling messages.In the exemplary embodiment, the transmitted data units are deleted fromqueue 40 after transmission.

V. Other Considerations for Forward Link Data Transmissions

An important consideration in the data communication system of thepresent invention is the accuracy of the C/I estimates for the purposeof selecting the data rate for future transmissions. In the exemplaryembodiment, the C/I measurements are performed on the pilot signalsduring the time interval when base stations 4 transmit pilot signals. Inthe exemplary embodiment, since only the pilot signals are transmittedduring this pilot time interval, the effects of multipath andinterference are minimal.

In other implementations of the present invention wherein the pilotsignals are transmitted continuously over an orthogonal code channel,similar to that for the IS-95 systems, the effect of multipath andinterference can distort the C/I measurements. Similarly, whenperforming the C/I measurement on the data transmissions instead of thepilot signals, multipath and interference can also degrade the C/Imeasurements. In both of these cases, when one base station 4 istransmitting to one mobile station 6, the mobile station 6 is able toaccurately measure the C/I of the forward link signal because no otherinterfering signals are present. However, when mobile station 6 is insoft handoff and receives the pilot signals from multiple base stations4, mobile station 6 is not able to discern whether or not base stations4 were transmitting data. In the worst case scenario, mobile station 6can measure a high C/I at a first time slot, when no base stations 4were transmitting data to any mobile station 6, and receive datatransmission at a second time slot, when all base stations 4 aretransmitting data at the same time slot. The C/I measurement at thefirst time slot, when all base stations 4 are idle, gives a falseindication of the forward link signal quality at the second time slotsince the status of the data communication system has changed. In fact,the actual C/I at the second time slot can be degraded to the point thatreliable decoding at the requested data rate is not possible.

The converse extreme scenario exists when a C/I estimate by mobilestation 6 is based on maximal interference. However, the actualtransmission occurs when only the selected base station is transmitting.In this case, the C/I estimate and selected data rate are conservativeand the transmission occurs at a rate lower than that which could bereliably decoded, thus reducing the transmission efficiency.

In the implementation wherein the C/I measurement is performed on acontinuous pilot signal or the traffic signal, the prediction of the C/Iat the second time slot based on the measurement of the C/I at the firsttime slot can be made more 22 accurate by three embodiments. In thefirst embodiment, data transmissions from base stations 4 are controlledso that base stations 4 do not constantly toggle between the transmitand idle states at successive time slots. This can be achieved byqueuing enough data (e.g., a predetermined number of information bits)before actual data transmission to mobile stations 6.

In the second embodiment, each base station 4 transmits a forwardactivity bit (hereinafter referred to as the FAC bit) which indicateswhether a transmission will occur at the next half frame. The use of theFAC bit is described in detail below. Mobile station 6 performs the C/Imeasurement taking into account the received FAC bit from each basestation 4.

In the third embodiment, which corresponds to the scheme wherein anindication of the link quality is transmitted to base station 4 andwhich uses a centralized scheduling scheme, the scheduling informationindicating which ones of base stations 4 transmitted data at each timeslot is made available to channel scheduler 48. Channel scheduler 48receives the C/I measurements from mobile stations 6 and can adjust theC/I measurements based on its knowledge of the presence or absence ofdata transmission from each base station 4 in the data communicationsystem. For example, mobile station 6 can measure the C/I at the firsttime slot when no adjacent base stations 4 are transmitting. Themeasured C/I is provided to channel scheduler 48. Channel scheduler 48knows that no adjacent base stations 4 transmitted data in the firsttime slot since none was scheduled by channel scheduler 48. Inscheduling data transmission at the second time slot, channel scheduler48 knows whether one or more adjacent base stations 4 will transmitdata. Channel scheduler 48 can adjust the C/I measured at the first timeslot to take into account the additional interference mobile station 6will receive in the second time slot due to data transmissions byadjacent base stations 4. Alternately, if the C/I is measured at thefirst time slot when adjacent base stations 4 are transmitting and theseadjacent base stations 4 are not transmitting at the second time slot,channel scheduler 48 can adjust the C/I measurement to take into accountthe additional information.

Another important consideration is to minimize redundantretransmissions. Redundant retransmissions can result from allowingmobile station 6 to select data transmission from different basestations 4 at successive time slots. The best C/I measurement can togglebetween two or more base stations 4 over successive time slots if mobilestation 6 measures approximately equal C/I for these base stations 4.The toggling can be due to deviations in the C/I measurements and/orchanges in the channel condition. Data transmission by different basestations 4 at successive time slots can result in a loss in efficiency.

The toggling problem can be addressed by the use of hysteresis. Thehysteresis can be implemented with a signal level scheme, a timingscheme, or a combination of the signal level and timing schemes. In theexemplary signal level scheme, the better C/I measurement of a differentbase station 4 in the active set is not selected unless it exceeds theC/I measurement of the current transmitting base station 4 by at leastthe hysteresis quantity. As an example, assume that the hysteresis is1.0 dB and that the C/I measurement of the first base station 4 is 3.5dB and the C/I measurement of the second base station 4 is 3.0 dB at thefirst time slot. At the next time slot, the second base station 4 is notselected unless its C/I measurement is at least 1.0 dB higher than thatof the first base station 4. Thus, if the C/I measurement of the firstbase station 4 is still 3.5 dB at the next time slot, the second basestation 4 is not selected unless its C/I measurement is at least 4.5 dB.

In the exemplary timing scheme, base station 4 transmits data packets tomobile station 6 for a predetermined number of time slots. Mobilestation 6 is not allowed to select a different transmitting base station4 within the predetermined number of time slots. Mobile station 6continues to measure the C/I of the current transmitting base station 4at each time slot and selects the data rate in response to the C/Imeasurement.

Yet another important consideration is the efficiency of the datatransmission. Referring to FIGS. 4E and 4F, each data packet format 410and 430 contains data and overhead bits. In the exemplary embodiment,the number of overhead bits is fixed for all data rates. At the highestdata rate, the percentage of overhead is small relative to the packetsize and the efficiency is high. At the lower data rates, the overheadbits can comprise a larger percentage of the packet. The inefficiency atthe lower data rates can be improved by transmitting variable lengthdata packets to mobile station 6. The variable length data packets canbe partitioned and transmitted to mobile station 6 over multiple timeslots. Preferably, the variable length data packets are transmitted tomobile station 6 over successive time slots to simplify the processing.The present invention is directed to the use of various packet sizes forvarious supported data rates to improve the overall transmissionefficiency.

VI. Forward Link Architecture

In the exemplary embodiment, base station 4 transmits at the maximumpower available to base station 4 and at the maximum data rate supportedby the data communication system to a single mobile station 6 at anygiven slot. The maximum data rate that can be supported is dynamic anddepends on the C/I of the forward link signal as measured by mobilestation 6. Preferably, base station 4 transmits to only one mobilestation 6 at any given time slot.

To facilitate data transmission, the forward link comprises four timemultiplexed channels: the pilot channel, power control channel, controlchannel, and traffic channel. The function and implementation of each ofthese channels are described below. In the exemplary embodiment, thetraffic and power control channels each comprises a number oforthogonally spread Walsh channels. In the present invention, thetraffic channel is used to transmit traffic data and paging messages tomobile stations 6. When used to transmit paging messages, the trafficchannel is also referred to as the control channel in thisspecification.

In the exemplary embodiment, the bandwidth of the forward link isselected to be 1.2288 MHz. This bandwidth selection allows the use ofexisting hardware components designed for a CDMA system which conformsto the IS-95 standard. However, the data communication system of thepresent invention can be adopted for use with different bandwidths toimprove capacity and/or to conform to system requirements. For example,a 5 MHz bandwidth can be utilized to increase the capacity. Furthermore,the bandwidths of the forward link and the reverse link can be different(e.g., 5 MHz bandwidth on the forward link and 1.2288 MHz bandwidth onthe reverse link) to more closely match link capacity with demand.

In the exemplary embodiment, the short PN_(I) and PN_(Q) codes are thesame length 2¹⁵ PN codes which are specified by the IS-95 standard. Atthe 1.2288 MHz chip rate, the short PN sequences repeat every 26.67 msec{26.67 msec=2¹⁵/1.2288×10⁶}. In the exemplary embodiment, the same shortPN codes are used by all base stations 4 within the data communicationsystem. However, each base station 4 is identified by a unique offset ofthe basic short PN sequences. In the exemplary embodiment, the offset isin increments of 64 chips. Other bandwidth and PN codes can be utilizedand are within the scope of the present invention.

VII. Forward Link Traffic Channel

A block diagram of the exemplary forward link architecture of thepresent invention is shown in FIG. 3A. The data is partitioned into datapackets and provided to CRC encoder 112. For each data packet, CRCencoder 112 generates frame check bits (e.g., the CRC parity bits) andinserts the code tail bits. The formatted packet from CRC encoder 112comprises the data, the frame check and code tail bits, and otheroverhead bits, which are described below. The formatted packet isprovided to encoder 114, which, in the exemplary embodiment, encodes thepacket in accordance with the encoding format disclosed in theaforementioned U.S. Pat. No. 5,933,462. Other encoding formats can alsobe used and are within the scope of the present invention. The encodedpacket from encoder 114 is provided to interleaver 116, which reordersthe code symbols in the packet. The interleaved packet is provided toframe puncture element 118, which removes a fraction of the packet inthe manner described below. The punctured packet is provided tomultiplier 120, which scrambles the data with the scrambling sequencefrom scrambler 122. Puncture element 118 and scrambler 122 are describedin detail below. The output from multiplier 120 comprises the scrambledpacket.

The scrambled packet is provided to variable rate controller 130, whichdemultiplexes the packet into K parallel inphase and quadraturechannels, where K is dependent on the data rate. In the exemplaryembodiment, the scrambled packet is first demultiplexed into the inphase(I) and quadrature (Q) streams. In the exemplary embodiment, the Istream comprises even indexed symbols and the Q stream comprises oddindexed symbol. Each stream is further demultiplexed into K parallelchannels such that the symbol rate of each channel is fixed for all datarates. The K channels of each stream are provided to Walsh cover element132, which covers each channel with a Walsh function to provideorthogonal channels. The orthogonal channel data are provided to gainelement 134, which scales the data to maintain a constanttotal-energy-per-chip (and hence constant output power) for all datarates. The scaled data from gain element 134 is provided to multiplexer(MUX) 160, which multiplexes the data with the preamble. The preamble isdiscussed in detail below. The output from MUX 160 is provided tomultiplexer (MUX) 162, which multiplexes the traffic data, the powercontrol bits, and the pilot data. The output of MUX 162 comprises the IWalsh channels and the Q Walsh channels.

A block diagram of the exemplary modulator used to modulate the data isillustrated in FIG. 3B. The I Walsh channels and Q Walsh channels areprovided to summers 212 a and 212 b, respectively, which sum the K Walshchannels to provide the signals I_(sum) and Q_(sum), respectively. TheI_(sum) and Q_(sum) signals are provided to complex multiplier 214.Complex multiplier 214 also receives the PN_I and PN_Q signals frommultipliers 236 a and 236 b, respectively, and multiplies the twocomplex inputs in accordance with the following equation:$\begin{matrix}\begin{matrix}{\left( {I_{mult} + {jQ}_{mult}} \right) = {\left( {I_{sum} + {jQ}_{sum}} \right) \cdot \left( {{PN\_ I} + {jPN\_ Q}} \right)}} \\{= {\left( {{I_{sum} \cdot {PN\_ I}} - {Q_{sum} \cdot {PN\_ Q}}} \right) +}} \\{{j\left( {{I_{sum} \cdot {PN\_ Q}} + {Q_{sum} \cdot {PN\_ I}}} \right)},}\end{matrix} & (2)\end{matrix}$where I_(mult) and Q_(mult) are the outputs from complex multiplier 214and j is the complex representation. The I_(mult) and Q_(mult) signalsare provided to filters 216 a and 216 b, respectively, which filters thesignals. The filtered signals from filters 216 a and 216 b are providedto multipliers 218 a and 218 b, respectively, which multiplies thesignals with the in-phase sinusoid COS(w_(c)t) and the quadraturesinusoid SIN(w_(c)t), respectively. The I modulated and Q modulatedsignals are provided to summer 220 which sums the signals to provide theforward modulated waveform S(t).

In the exemplary embodiment, the data packet is spread with the long PNcode and the short PN codes. The long PN code scrambles the packet suchthat only the mobile station 6 for which the packet is destined is ableto descramble the packet. In the exemplary embodiment, the pilot andpower control bits and the control channel packet are spread with theshort PN codes but not the long PN code to allow all mobile stations 6to receive these bits. The long PN sequence is generated by long codegenerator 232 and provided to multiplexer (MUX) 234. The long PN maskdetermines the offset of the long PN sequence and is uniquely assignedto the destination mobile station 6. The output from MUX 234 is the longPN sequence during the data portion of the transmission and zerootherwise (e.g., during the pilot and power control portion). The gatedlong PN sequence from MUX 234 and the short PN_(I) and PN_(Q) sequencesfrom short code generator 238 are provided to multipliers 236 a and 236b, respectively, which multiply the two sets of sequences to form thePN_I and PN_Q signals, respectively. The PN_I and PN_Q signals areprovided to complex multiplier 214.

The block diagram of the exemplary traffic channel shown in FIGS. 3A and3B is one of numerous architectures, which support data encoding andmodulation on the forward link. Other architectures, such as thearchitecture for the forward link traffic channel in the CDMA system,which conforms to the IS-95 standard, can also be utilized and arewithin the scope of the present invention.

In the exemplary embodiment, the data rates supported by base stations 4are predetermined and each supported data rate is assigned a unique rateindex. Mobile station 6 selects one of the supported data rates based onthe C/I measurement. Since the requested data rate needs to be sent to abase station 4 to direct that base station 4 to transmit data at therequested data rate, a trade off is made between the number of supporteddata rates and the number of bits needed to identify the requested datarate. In the exemplary embodiment, the number of supported data rates isseven and a 3-bit rate index is used to identify the requested datarate. An exemplary definition of the supported data rates is illustratedin Table 1. Different definition of the supported data rates can becontemplated and are within the scope of the present invention.

In the exemplary embodiment, the minimum data rate is 38.4 Kbps and themaximum data rate is 2.4576 Mbps. The minimum data rate is selectedbased on the worse case C/I measurement in the system, the processinggain of the system, the design of the error correcting codes, and thedesired level of performance. In the exemplary embodiment, the supporteddata rates are chosen such that the difference between successivesupported data rates is 3 dB. The 3 dB increment is a compromise amongseveral factors which include the accuracy of the C/I measurement thatcan be achieved by mobile station 6, the losses (or inefficiencies)which results from the quantization of the data rates based on the C/Imeasurement, and the number of bits (or the bit rate) needed to transmitthe requested data rate from mobile station 6 to base station 4. Moresupported data rates requires more bits to identify the requested datarate but allows for more efficient use of the forward link because ofsmaller quantization error between the calculated maximum data rate andthe supported data rate. The present invention is directed to the use ofany number of supported data rates and other data rates than thoselisted in Table 1. TABLE 1 Traffic Channel Parameters Parameter DataRates Units 38.4 76.8 153.6 307.2 614.4 1228.8 2457.6 Kbps Databit/packet 1024 1024 1024 1024 1024 2048 2048 bits Packet length 26.6713.33 6.67 3.33 1.67 1.67 0.83 msec Slots/packet 16 8 4 2 1 1 0.5 slotsPacket/transmission 1 1 1 1 1 1 2 packets Slots/transmission 16 8 4 2 11 1 slots Walsh symbol rate 153.6 307.2 614.4 1228.8 2457.6 2457.64915.2 Ksps Walsh channel/ 1 2 4 8 16 16 16 channels QPSK phaseModulator rate 76.8 76.8 76.8 76.8 76.8 76.8 76.8¹ ksps PN chips/databit 32 16 8 4 2 1 0.5 chips/bit PN chip rate 1228.8 1228.8 1228.8 1228.81228.8 1228.8 1228.8 Kcps Modulation format QPSK QPSK QPSK QPSK QPSKQPSK QAM¹ Rate index 0 1 2 3 4 5 6Note:¹16-QAM modulation

A diagram of the exemplary forward link frame structure of the presentinvention is illustrated in FIG. 4A. The traffic channel transmission ispartitioned into frames which, in the exemplary embodiment, are definedas the length of the short PN sequences or 26.67 msec. Each frame cancarry control channel information addressed to all mobile stations 6(control channel frame), traffic data addressed to a particular mobilestation 6 (traffic frame), or can be empty (idle frame). The content ofeach frame is determined by the scheduling performed by the transmittingbase station 4. In the exemplary embodiment, each frame comprises 16time slots, with each time slot having a duration of 1.667 msec. A timeslot of 1.667 msec is adequate to enable mobile station 6 to perform theC/I measurement of the forward link signal. A time slot of 1.667 msecalso represents a sufficient amount of time for efficient packet datatransmission. In the exemplary embodiment, each time slot is furtherpartitioned into four-quarter slots.

In the present invention, each data packet is transmitted over one ormore time slots as shown in Table 1. In the exemplary embodiment, eachforward link data packet comprises 1024 or 2048 bits. Thus, the numberof time slots required to transmit each data packet is dependent on thedata rate and ranges from 16 time slots for the 38.4 Kbps rate to 1 timeslot for the 1.2288 Mbps rate and higher.

An exemplary diagram of the forward link slot structure of the presentinvention is shown in FIG. 4B. In the exemplary embodiment, each slotcomprises three of the four time multiplexed channels, the trafficchannel, the control channel, the pilot channel, and the power controlchannel. In the exemplary embodiment, the pilot and power controlchannels are transmitted in two pilot and power control bursts, whichare located at the same positions in each time slot. The pilot and powercontrol bursts are described in detail below.

In the exemplary embodiment, the interleaved packet from interleaver 116is punctured to accommodate the pilot and power control bursts. In theexemplary embodiment, each interleaved packet comprises 4096 codesymbols and the first 512 code symbols are punctured, as shown in FIG.4D. The remaining code symbols are skewed in time to align to thetraffic channel transmission intervals.

The punctured code symbols are scrambled to randomize the data prior toapplying the orthogonal Walsh cover. The randomization limits thepeak-to-average envelope on the modulated waveform S(t). The scramblingsequence can be generated with a linear feedback shift register, in amanner known in the art. In the exemplary embodiment, scrambler 122 isloaded with the LC state at the start of each slot. In the exemplaryembodiment, the clock of scrambler 122 is synchronous with the clock ofinterleaver 116 but is stalled during the pilot and power controlbursts.

In the exemplary embodiment, the forward Walsh channels (for the trafficchannel and power control channel) are orthogonally spread with 16-bitWalsh covers at the fixed chip rate of 1.2288 Mcps. The number ofparallel orthogonal channels K per in-phase and quadrature signal is afunction of the data rate, as shown in Table 1. In the exemplaryembodiment, for lower data rates, the in-phase and quadrature Walshcovers are chosen to be orthogonal sets to minimize cross-talk to thedemodulator phase estimate errors. For example, for 16 Walsh channels,an exemplary Walsh assignment is W₀ through W₇ for the in-phase signaland W₈ through W₁₅ for the quadrature signal.

In the exemplary embodiment, QPSK modulation is used for data rates of1.2288 Mbps and lower. For QPSK modulation, each Walsh channel comprisesone bit. In the exemplary embodiment, at the highest data rate of 2.4576Mbps, 16-QAM is used and the scrambled data is demultiplexed into 32parallel streams which are each 2-bit wide, 16 parallel streams for theinphase signal and 16 parallel streams for the quadrature signal. In theexemplary embodiment, the LSB of each 2-bit symbol is the earlier symboloutput from interleaver 116. In the exemplary embodiment, the QAMmodulation inputs of (0, 1, 3, 2) map to modulation values of (+3, +1,−1, −3), respectively. The use of other modulation schemes, such as manyphase shift keying PSK, can be contemplated and are within the scope ofthe present invention.

The in-phase and quadrature Walsh channels are scaled prior tomodulation to maintain a constant total transmit power which isindependent of the data rate. The gain settings are normalized to aunity reference equivalent to unmodulated BPSK. The normalized channelgains G as a function of the number of Walsh channels (or data rate) areshown in Table 2. Also listed in Table 2 is the average power per Walshchannel (inphase or quadrature) such that the total normalized power isequal to unity. Note that the channel gain for 16-QAM accounts for thefact that the normalized energy per Walsh chip is 1 for QPSK and 5 for16-QAM. TABLE 2 Traffic Channel Orthogonal Channel Gains PunctureDuration Number of Walsh Average Data Rate Walsh Channel Power per(Kbps) Channels K Modulation Gain G Channel P_(k) 38.4 1 QPSK 1/{squareroot over (2)} ½  76.8 2 QPSK ½ ¼  153.6 4 QPSK 1/2{square root over(2)} ⅛  307.2 8 QPSK ¼ 1/16 614.4 16 QPSK 1/4{square root over (2)} 1/321228.8 16 QPSK 1/4{square root over (2)} 1/32 2457.6 16 16-QAM1/4{square root over (10)} 1/32

In the present invention, a preamble is punctured into each trafficframe to assist mobile station 6 in the synchronization with the firstslot of each variable rate transmission. In the exemplary embodiment,the preamble is an all-zero sequence, which, for a traffic frame, isspread with the long PN code but, for a control channel frame, is notspread with the long PN code. In the exemplary embodiment, the preambleis unmodulated BPSK which is orthogonally spread with Walsh cover W₁.The use of a single orthogonal channel minimizes the peak-to-averageenvelope. Also, the use of a non-zero Walsh cover W₁ minimizes falsepilot detection since, for traffic frames, the pilot is spread withWalsh cover W₀ and both the pilot and the preamble are not spread withthe long PN code.

The preamble is multiplexed into the traffic channel stream at the startof the packet for a duration that is a function of the data rate. Thelength of the preamble is such that the preamble overhead isapproximately constant for all data rates while minimizing theprobability of false detection. A summary of the preamble as a functionof data rates is shown in Table 3. Note that the preamble comprises 3.1percent or less of a data packet. TABLE 3 Preamble Parameters PreamblePuncture Duration Data Rate Walsh (Kbps) Symbols PN chips Overhead 38.432 512 1.6% 76.8 16 256 1.6% 153.6 8 128 1.6% 307.2 4 64 1.6% 614.4 3 482.3% 1228.8 4 64 3.1% 2457.6 2 32 3.1%VIII. Forward Link Traffic Frame Format

In the exemplary embodiment, each data packet is formatted by theadditions of frame check bits, code tail bits, and other control fields.In this specification, an octet is defined as 8 information bits and adata unit is a single octet and comprises 8 information bits.

In the exemplary embodiment, the forward link supports two data packetformats, which are illustrated in FIGS. 4E and 4F. Packet format 410comprises five fields and packet format 430 comprises nine fields.Packet format 410 is used when the data packet to be transmitted tomobile station 6 contains enough data to completely fill all availableoctets in DATA field 418. If the amount of data to be transmitted isless than the available octets in DATA field 418, packet format 430 isused. The unused octets are padded with all zeros and designated asPADDING field 446.

In the exemplary embodiment, frame check sequence (FCS) fields 412 and432 contain the CRC parity bits which are generated by CRC generator 112(see FIG. 3A) in accordance with a predetermined generator polynomial.In the exemplary embodiment, the CRC polynomial is g(x)=x¹⁶+x¹²+x⁵+1,although other polynomials can be used and are within the scope of thepresent invention. In the exemplary embodiment, the CRC bits arecalculated over the FMT, SEQ, LEN, DATA, and PADDING fields. Thisprovides error detection over all bits, except the code tail bits inTAIL fields 420 and 448, transmitted over the traffic channel on theforward link. In the alternative embodiment, the CRC bits are calculatedonly over the DATA field. In the exemplary embodiment, FCS fields 412and 432 contain 16 CRC parity bits, although other CRC generatorsproviding different number of parity bits can be used and are within thescope of the present invention. Although FCS fields 412 and 432 of thepresent invention has been described in the context of CRC parity bits,other frame check sequences can be used and are within the scope of thepresent invention. For example, a check sum can be calculated for thepacket and provided in the FCS field.

In the exemplary embodiment, frame format (FMT) fields 414 and 434contain one control bit which indicates whether the data frame containsonly data octets (packet format 410) or data and padding octets and zeroor more messages (packet format 430). In the exemplary embodiment, a lowvalue for FMT field 414 corresponds to packet format 410. Alternatively,a high value for FMT field 434 corresponds to packet format 430.

Sequence number (SEQ) fields 416 and 442 identify the first data unit indata fields 418 and 444, respectively. The sequence number allows datato be transmitted out of sequence to mobile station 6, e.g., forretransmission of packets which have been received in error. Theassignment of the sequence number at the data unit level eliminates theneed for frame fragmentation protocol for retransmission. The sequencenumber also allows mobile station 6 to detect duplicate data units. Uponreceipt of the FMT, SEQ, and LEN fields, mobile station 6 is able todetermine which data units have been received at each time slot withoutthe use of special signaling messages.

The number of bits assigned to represent the sequence number isdependent on the maximum number of data units, which can be transmittedin one time slot and the worse case data retransmission delays. In theexemplary embodiment, each data unit is identified by a 24-bit sequencenumber. At the 2.4576 Mbps data rate, the maximum number of data units,which can be transmitted at each slot is approximately 256. Eight bitsare required to identify each of the data units. Furthermore, it can becalculated that the worse case data retransmission delays are less than500 msec. The retransmission delays include the time necessary for aNACK message by mobile station 6, retransmission of the data, and thenumber of retransmission attempts caused by the worse case burst errorruns. Therefore, 24 bits allows mobile station 6 to properly identifythe data units being received without ambiguity. The number of bits inSEQ fields 416 and 442 can be increased or decreased, depending on thesize of DATA field 418 and the retransmission delays. The use ofdifferent number of bits for SEQ fields 416 and 442 are within the scopeof the present invention.

When base station 4 has less data to transmit to mobile station 6 thanthe space available in DATA field 418, packet format 430 is used. Packetformat 430 allows base station 4 to transmit any number of data units,up to the maximum number of available data units, to mobile station 6.In the exemplary embodiment, a high value for FMT field 434 indicatesthat base station 4 is transmitting packet format 430. Within packetformat 430, LEN field 440 contains the value of the number of data unitsbeing transmitted in that packet. In the exemplary embodiment, LEN field440 is 8 bits in length since DATA field 444 can range from 0 to 255octets.

DATA fields 418 and 444 contain the data to be transmitted to mobilestation 6. In the exemplary embodiment, for packet format 410, each datapacket comprises 1024 bits of which 992 are data bits. However, variablelength data packets can be used to increase the number of informationbits and are within the scope of the present invention. For packetformat 430, the size of DATA field 444 is determined by LEN field 440.

In the exemplary embodiment, packet format 430 can be used to transmitzero or more signaling messages. Signaling length (SIG LEN) field 436contains the length of the subsequent signaling messages, in octets. Inthe exemplary embodiment, SIG LEN field 436 is 8 bits in length.SIGNALING field 438 contains the signaling messages. In the exemplaryembodiment, each signaling message comprises a message identification(MESSAGE ID) field, a message length (LEN) field, and a message payload,as described below.

PADDING field 446 contains padding octets which, in the exemplaryembodiment, are set to 0x00 (hex). PADDING field 446 is used becausebase station 4 may have fewer data octets to transmit to mobile station6 than the number of octets available in DATA field 418. When thisoccurs, PADDING field 446 contains enough padding octets to fill theunused data field. PADDING field 446 is variable length and depends onthe length of DATA field 444.

The last field of packet formats 410 and 430 is TAIL fields 420 and 448,respectively. TAIL fields 420 and 448 contain the zero (0x0) code tailbits which are used to force encoder 114 (see FIG. 3A) into a knownstate at the end of each data packet. The code tail bits allow encoder114 to succinctly partition the packet such that only bits from onepacket are used in the encoding process. The code tail bits also allowthe decoder within mobile station 6 to determine the packet boundariesduring the decoding process. The number of bits in TAIL fields 420 and448 depends on the design of encoder 114. In the exemplary embodiment,TAIL fields 420 and 448 are long enough to force encoder 114 to a knownstate.

The two packet formats described above are exemplary formats which canbe used to facilitate transmission of data and signaling messages.Various other packet formats can be create to meet the needs of aparticular communication system. Also, a communication system can bedesigned to accommodate more than the two packet formats describedabove.

IX. Forward Link Control Channel Frame

In the present invention, the traffic channel is also used to transmitmessages from base station 4 to mobile stations 6. The types of messagestransmitted include: (1) handoff direction messages, (2) paging messages(e.g. to page a specific mobile station 6 that there is data in thequeue for that mobile station 6), (3) short data packets for a specificmobile station 6, and (4) ACK or NACK messages for the reverse link datatransmissions (to be described later herein). Other types of messagescan also be transmitted on the control channel and are within the scopeof the present invention. Upon completion of the call set up stage,mobile station 6 monitors the control channel for paging messages andbegins transmission of the reverse link pilot signal.

In the exemplary embodiment, the control channel is time multiplexedwith the traffic data on the traffic channel, as shown in FIG. 4A.Mobile stations 6 identify the control message by detecting a preamblewhich has been covered with a predetermined PN code. In the exemplaryembodiment, the control messages are transmitted at a fixed rate, whichis determined by mobile station 6 during acquisition. In the preferredembodiment, the data rate of the control channel is 76.8 Kbps.

The control channel transmits messages in control channel capsules. Thediagram of an exemplary control channel capsule is shown in FIG. 4G. 1nthe exemplary embodiment, each capsule comprises preamble 462, thecontrol payload, and CRC parity bits 474. The control payload comprisesone or more messages and, if necessary, padding bits 472. Each messagecomprises message identifier (MSG ID) 464, message length (LEN) 466,optional address (ADDR) 468 (e.g., if the message is directed to aspecific mobile station 6), and message payload 470. In the exemplaryembodiment, the messages are aligned to octet boundaries. The exemplarycontrol channel capsule illustrated in FIG. 4G comprises two broadcastmessages intended for all mobile stations 6 and one message directed ata specific mobile station 6. MSG ID field 464 determines whether or notthe message requires an address field (e.g., whether it is a broadcastor a specific message).

X. Forward Link Pilot Channel

In the present invention, a forward link pilot channel provides a pilotsignal that is used by mobile stations 6 for initial acquisition, phaserecovery, timing recovery, and ratio combining. These uses are similarto that of the CDMA communication systems that conform to IS-95standard. In the exemplary embodiment, mobile stations 6 to perform theC/I measurement also use the pilot signal.

The exemplary block diagram of the forward link pilot channel of thepresent invention is shown in FIG. 3A. The pilot data comprises asequence of all zeros (or all ones), which is provided to multiplier156. Multiplier 156 covers the pilot data with Walsh code W₀. SinceWalsh code W₀ is a sequence of all zeros, the output of multiplier 156is the pilot data. The pilot data is time multiplexed by MUX 162 andprovided to the I Walsh channel, which is spread by the short PN_(I)code within complex multiplier 214 (see FIG. 3B). In the exemplaryembodiment, the pilot data is not spread with the long PN code, which isgated off during the pilot burst by MUX 234, to allow reception by allmobile stations 6. The pilot signal is thus an unmodulated BPSK signal.

A diagram illustrating the pilot signal is shown in FIG. 4B. In theexemplary embodiment, each time slot comprises two pilot bursts 306 aand 306 b, which occur at the end of the first and third quarters of thetime slot. In the exemplary embodiment, each pilot burst 306 is 64 chipsin duration (Tp=64 chips). In the absence of traffic data or controlchannel data, base station 4 only transmits the pilot and power controlbursts, resulting in a discontinuous waveform bursting at the periodicrate of 1200 Hz. The pilot modulation parameters are tabulated in Table4.

XI. Reverse Link Power Control

In the present invention, the forward link power control channel is usedto send the power control command which is used to control the transmitpower of the reverse link transmission from remote station 6. On thereverse link, each transmitting mobile station 6 acts as a source ofinterference to all other mobile stations 6 in the network. To minimizeinterference on the reverse link and maximize capacity, the transmitpower of each mobile station 6 is controlled by two power control loops.In the exemplary embodiment, the power control loops are similar to thatof the CDMA system disclosed in detail in U.S. Pat. No. 5,056,109,entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN ACDMA CELLULAR MOBILE TELEPHONE SYSTEM,” assigned to the assignee of thepresent invention and incorporated by reference herein. Other powercontrol mechanism can also be contemplated and are within the scope ofthe present invention.

The first power control loop adjusts the transmit power of mobilestation 6 such that the reverse link signal quality is maintained at aset level. The signal quality is measured as theenergy-per-bit-to-noise-plus-interference ratio E_(b)/I_(o) of thereverse link signal received at base station 4. The set level isreferred to as the E_(b)/I_(o) set point. The second power control loopadjusts the set point such that the desired level of performance, asmeasured by the frame-error-rate (FER), is maintained. Power control iscritical on the reverse link because the transmit power of each mobilestation 6 is an interference to other mobile stations 6 in thecommunication system. Minimizing the reverse link transmit power reducesthe interference and increases the reverse link capacity.

Within the first power control loop, the E_(b)/I_(o) of the reverse linksignal is measured at base station 4. Base station 4 then compares themeasured E_(b)/I_(o) with the set point. If the measured E_(b)/I_(o) isgreater than the set point, base station 4 transmits a power controlmessage to mobile station 6 to decrease the transmit power.Alternatively, if the measured E_(b)/I_(o) is below the set point, basestation 4 transmits a power control message to mobile station 6 toincrease the transmit power. In the exemplary embodiment, the powercontrol message is implemented with one power control bit. In theexemplary embodiment, a high value for the power control bit commandsmobile station 6 to increase its transmit power and a low value commandsmobile station 6 to decrease its transmit power.

In the present invention, the power control bits for all mobile stations6 in communication with each base station 4 are transmitted on the powercontrol channel. In the exemplary embodiment, the power control channelcomprises up to 32 orthogonal channels, which are spread with the 16-bitWalsh covers. Each Walsh channel transmits one reverse power control(RPC) bit or one FAC bit at periodic intervals. Each active mobilestation 6 is assigned an RPC index, which defines the Walsh cover andQPSK modulation phase (e.g., inphase or quadrature) for transmission ofthe RPC bit stream destined for that mobile station 6. In the exemplaryembodiment, the RPC index of 0 is reserved for the FAC bit.

The exemplary block diagram of the power control channel is shown inFIG. 3A. The RPC bits are provided to symbol repeater 150, which repeatseach RPC bit a predetermined number of times. The repeated RPC bits areprovided to Walsh cover element 152, which covers the bits with theWalsh covers corresponding to the RPC indices. The covered bits areprovided to gain element 154 which scales the bits prior to modulationso as to maintain a constant total transmit power. In the exemplaryembodiment, the gains of the RPC Walsh channels are normalized so thatthe total RPC channel power is equal to the total available transmitpower. The gains of the Walsh channels can be varied as a function oftime for efficient utilization of the total base station transmit powerwhile maintaining reliable RPC transmission to all active mobilestations 6. In the exemplary embodiment, the Walsh channel gains ofinactive mobile stations 6 are set to zero. Automatic power control ofthe RPC Walsh channels is possible using estimates of the forward linkquality measurement from the corresponding DRC channel from mobilestations 6. The scaled RPC bits from gain element 154 are provided toMUX 162.

In the exemplary embodiment, the RPC indices of 0 through 15 areassigned to Walsh covers W₀ through W₁₅, respectively, and aretransmitted around the first pilot burst within a slot (RPC bursts 304in FIG. 4C). The RPC indices of 16 through 31 are assigned to Walshcovers W₀ through W₁₅, respectively, and are transmitted around thesecond pilot burst within a slot (RPC bursts 308 in FIG. 4C). In theexemplary embodiment, the RPC bits are BPSK modulated with the evenWalsh covers (e.g., W₀, W₂, W₄, etc.) modulated on the inphase signaland the odd Walsh covers (e.g., W₁, W₃, W₅, etc.) modulated on thequadrature signal. To reduce the peak-to-average envelope, it ispreferable to balance the inphase and quadrature power. Furthermore, tominimize cross-talk due to demodulator phase estimate error, it ispreferable to assign orthogonal covers to the inphase and quadraturesignals.

In the exemplary embodiment, up to 31 RPC bits can be transmitted on 31RPC Walsh channels in each time slot. In the exemplary embodiment, 15RPC bits are transmitted on the first half slot and 16 RPC bits aretransmitted on the second half slot. The RPC bits are combined bysummers 212 (see FIG. 3B) and the composite waveform of the powercontrol channel is as shown is in FIG. 4C.

A timing diagram of the power control channel is illustrated in FIG. 4B.In the exemplary embodiment, the RPC bit rate is 600 bps, or one RPC bitper time slot. Each RPC bit is time multiplexed and transmitted over twoRPC bursts (e.g., RPC bursts 304 a and 304 b), as shown in FIGS. 4B and4C. In the exemplary embodiment, each RPC burst is 32 PN chips (or 2Walsh symbols) in width (Tpc=32 chips) and the total width of each RPCbit is 64 PN chips (or 4 Walsh symbols). Other RPC bit rates can beobtained by changing the number of symbol repetition. For example, anRPC bit rate of 1200 bps (to support up to 63 mobile stations 6simultaneously or to increase the power control rate) can be obtained bytransmitting the first set of 31 RPC bits on RPC bursts 304 a and 304 band the second set of 32 RPC bits on RPC bursts 308 a and 308 b. In thiscase, all Walsh covers are used in the inphase and quadrature signals.The modulation parameters for the RPC bits are summarized in Table 4.TABLE 4 Pilot and Power Control Modulation Parameters Parameter RPC FACPilot Units Rate 600 75 1200 Hz Modulation format QPSK QPSK BPSKDuration of control bit 64 1024 64 PN chips Repeat 4 64 4 symbols

The power control channel has a bursty nature since the number of mobilestations 6 in communication with each base station 4 can be less thanthe number of available RPC Walsh channels. In this situation, some RPCWalsh channels are set to zero by proper adjustment of the gains of gainelement 154.

In the exemplary embodiment, the RPC bits are transmitted to mobilestations 6 without coding or interleaving to minimize processing delays.Furthermore, the erroneous reception of the power control bit is notdetrimental to the data communication system of the present inventionsince the error can be corrected in the next time slot by the powercontrol loop.

In the present invention, mobile stations 6 can be in soft handoff withmultiple base stations 4 on the reverse link. The method and apparatusfor the reverse link power control for mobile station 6 in soft handoffis disclosed in the aforementioned U.S. Pat. No. 5,056,109. Mobilestation 6 in soft handoff monitors the RPC Walsh channel for each basestation 4 in the active set and combines the RPC bits in accordance withthe method disclosed in the aforementioned U.S. Pat. No. 5,056,109. Inthe first embodiment, mobile station 6 performs the logic OR of the downpower commands. Mobile station 6 decreases the transmit power if any oneof the received RPC bits commands mobile station 6 to decrease thetransmit power. In the second embodiment, mobile station 6 in softhandoff can combine the soft decisions of the RPC bits before making ahard decision. Other embodiments for processing the received RPC bitscan be contemplated and are within the scope of the present invention.

In the present invention, the FAC bit indicates to mobile stations 6whether or not the traffic channel of the associated pilot channel willbe transmitting on the next half frame. The use of the FAC bit improvesthe C/I estimate by mobile stations 6, and hence the data rate request,by broadcasting the knowledge of the interference activity. In theexemplary embodiment, the FAC bit only changes at half frame boundariesand is repeated for eight successive time slots, resulting in a bit rateof 75 bps. The parameters for the FAC bit is listed in Table 4.

Using the FAC bit, mobile stations 6 can compute the C/I measurement asfollows: $\begin{matrix}{{\left( \frac{C}{I} \right)_{i} = \frac{C_{i}}{I - {\sum\limits_{j \neq i}{\left( {1 - \alpha_{j}} \right)C_{j}}}}},} & (3)\end{matrix}$where (C/I)_(i) is the C/I measurement of the i^(th) forward linksignal, C_(i) is the total received power of the i^(th) forward linksignal, C_(j) is the received power of the j^(th) forward link signal, Iis the total interference if all base stations 4 are transmitting, α_(j)is the FAC bit of the j^(th) forward link signal and can be 0 or 1depending on the FAC bit.XII. Reverse Link Data Transmission

In the present invention, the reverse link supports variable rate datatransmission. The variable rate provides flexibility and allows mobilestations 6 to transmit at one of several data rates, depending on theamount of data to be transmitted to base station 4. In the exemplaryembodiment, mobile station 6 can transmit data at the lowest data rateat any time. In the exemplary embodiment, data transmission at higherdata rates requires a grant by base station 4. This implementationminimizes the reverse link transmission delay while providing efficientutilization of the reverse link resource.

An exemplary illustration of the flow diagram of the reverse link datatransmission of the present invention is shown in FIG. 8. Initially, atslot n, mobile station 6 performs an access probe, as described in theaforementioned U.S. Pat. No. 5,289,527, to establish the lowest ratedata channel on the reverse link at block 802. In the same slot n, basestation 4 demodulates the access probe and receives the access messageat block 804. Base station 4 grants the request for the data channeland, at slot n+2, transmits the grant and the assigned RPC index on thecontrol channel, at block 806. At slot n+2, mobile station 6 receivesthe grant and is power controlled by base station 4, at block 808.Beginning at slot n+3, mobile station 6 starts transmitting the pilotsignal and has immediate access to the lowest rate data channel on thereverse link.

If mobile station 6 has traffic data and requires a high rate datachannel, mobile station 6 can initiate the request at block 810. At slotn+3, base station 4 receives the high speed data request, at block 812.At slot n+5, base station 4 transmits the grant on the control channel,at block 814. At slot n+5, mobile station 6 receives the grant at block816 and begins high speed data transmission on the reverse link startingat slot n+6, at block 818.

XIII. Reverse Link Architecture

In the data communication system of the present invention, the reverselink transmission differs from the forward link transmission in severalways. On the forward link, data transmission typically occurs from onebase station 4 to one mobile station 6. However, on the reverse link,each base station 4 can concurrently receive data transmissions frommultiple mobile stations 6. In the exemplary embodiment, each mobilestation 6 can transmit at one of several data rates depending on theamount of data to be transmitted to base station 4. This system designreflects the asymmetric characteristic of data communication.

In the exemplary embodiment, the time base unit on the reverse link isidentical to the time base unit on the forward link. In the exemplaryembodiment, the forward link and reverse link data transmissions occurover time slots, which are 1.667 msec in duration. However, since datatransmission on the reverse link typically occurs at a lower data rate,a longer time base unit can be used to improve efficiency.

In the exemplary embodiment, the reverse link supports two channels: thepilot/DRC channel and the data channel. The function and implementationof each of this channel are described below. The pilot/DRC channel isused to transmit the pilot signal and the DRC messages and the datachannel is used to transmit traffic data.

A diagram of the exemplary reverse link frame structure of the presentinvention is illustrated in FIG. 7A. In the exemplary embodiment, thereverse link frame structure is similar to the forward link framestructure shown in FIG. 4A. However, on the reverse link, the pilot/DRCdata and traffic data are transmitted concurrently on the in-phase andquadrature channels.

In the exemplary embodiment, mobile station 6 transmits a DRC message onthe pilot/DRC channel at each time slot whenever mobile station 6 isreceiving high speed data transmission. Alternatively, when mobilestation 6 is not receiving high speed data transmission, the entire sloton the pilot/DRC channel comprises the pilot signal. The pilot signal isused by the receiving base station 4 for a number of functions: as anaid to initial acquisition, as a phase reference for the pilot/DRC andthe data channels, and as the source for the closed loop reverse linkpower control.

In the exemplary embodiment, the bandwidth of the reverse link isselected to be 1.2288 MHz. This bandwidth selection allows the use ofexisting hardware designed for a CDMA system which conforms to the IS-95standard. However, other bandwidths can be utilized to increase capacityand/or to conform to system requirements. In the exemplary embodiment,the same long PN code and short PN_(I) and PN_(Q) codes as thosespecified by the IS-95 standard are used to spread the reverse linksignal. In the exemplary embodiment, the reverse link channels aretransmitted using QPSK modulation. Alternatively, OQPSK modulation canbe used to minimize the peak-to-average amplitude variation of themodulated signal which can result in improved performance. The use ofdifferent system bandwidth, PN codes, and modulation schemes can becontemplated and are within the scope of the present invention.

In the exemplary embodiment, the transmit power of the reverse linktransmissions on the pilot/DRC channel and the data channel arecontrolled such that the E_(b)/I_(o) of the reverse link signal, asmeasured at base station 4, is maintained at a predetermined E_(b)/I_(o)set point as discussed in the aforementioned U.S. Pat. No. 5,506,109.The power control is maintained by base stations 4 in communication withthe mobile station 6 and the commands are transmitted as the RPC bits asdiscussed above.

XIV. Reverse Link Data Channel

A block diagram of the exemplary reverse link architecture of thepresent invention is shown in FIG. 6. The data is partitioned into datapackets and provided to encoder 612. For each data packet, encoder 612generates the CRC parity bits, inserts the code tail bits, and encodesthe data. In the exemplary embodiment, encoder 612 encodes the packet inaccordance with the encoding format disclosed in the aforementioned U.S.Pat. No. 5,933,462. Other encoding formats can also be used and arewithin the scope of the present invention. The encoded packet fromencoder 612 is provided to block interleaver 614, which reorders thecode symbols in the packet. The interleaved packet is provided tomultiplier 616, which covers the data with the Walsh cover and providesthe covered data to gain element 618. Gain element 618 scales the datato maintain a constant energy-per-bit Eb regardless of the data rate.The scaled data from gain element 618 is provided to multipliers 650band 650d, which spread the data with the PN_Q and PN_I sequences,respectively. The spread data from multipliers 650 b and 650 d areprovided to filters 652 b and 652 d, respectively, which filter thedata. The filtered signals from filters 652 a and 652 b are provided tosummer 654 a and the filtered signals from filter 652 c and 652 d areprovided to summer 654 b. Summers 654 sum the signals from the datachannel with the signals from the pilot/DRC channel. The outputs ofsummers 654 a and 654 b comprise IOUT and QOUT, respectively, which aremodulated with the in-phase sinusoid COS(w_(c)t) and the quadraturesinusoid SIN(w_(c)t), respectively (as in the forward link), and summed(not shown in FIG. 6). In the exemplary embodiment, the traffic data istransmitted on both the inphase and quadrature phase of the sinusoid.

In the exemplary embodiment, the data is spread with the long PN codeand the short PN codes. The long PN code scrambles the data such thatthe receiving base station 4 is able to identify the transmitting mobilestation 6. The short PN code spreads the signal over the systembandwidth. The long PN sequence is generated by long code generator 642and provided to multipliers 646. The short PN_(I) and PN_(Q) sequencesare generated by short code generator 644 and also provided tomultipliers 646 a and 646 b, respectively, which multiply the two setsof sequences to form the PN_I and PN_Q signals, respectively.Timing/control circuit 640 provides the timing reference.

The exemplary block diagram of the data channel architecture as shown inFIG. 6 is one of numerous architectures which support data encoding andmodulation on the reverse link. For high rate data transmission, anarchitecture similar to that of the forward link utilizing multipleorthogonal channels can also be used. Other architectures, such as thearchitecture for the reverse link traffic channel in the CDMA systemwhich conforms to the IS-95 standard, can also be contemplated and arewithin the scope of the present invention.

In the exemplary embodiment, the reverse link data channel supports fourdata rates which are tabulated in Table 5. Additional data rates and/ordifferent data rates can be supported and are within the scope of thepresent invention. In the exemplary embodiment, the packet size for thereverse link is dependent on the data rate, as shown in Table 5. Asdescribed in the aforementioned U.S. Pat. No. 5,933,462, improveddecoder performance can be obtained for larger packet sizes. Thus,different packet sizes than those listed in Table 5 can be utilized toimprove performance and are within the scope of the present invention.In addition, the packet size can be made a parameter, which isindependent of the data rate. TABLE 5 Pilot and Power Control ModulationParameters Parameter Data rates Units 9.6 19.2 38.4 76.8 Kbps Frameduration 26.66 26.66 13.33 13.33 msec Data packet length 245 491 4911003 bits CRC length 16 16 16 16 bits Code tail bits 5 5 5 5 bits Totalbits/packet 256 512 512 1024 bits Encoded packet length 1024 2048 20484096 symbols Walsh symbol length 32 16 8 4 chips Request required no yesyes yes

As shown in Table 5, the reverse link supports a plurality of datarates. In the exemplary embodiment, the lowest data rate of 9.6K bps isallocated to each mobile station 6 upon registration with base station4. In the exemplary embodiment, mobile stations 6 can transmit data onthe lowest rate data channel at any time slot without having to requestpermission from base station 4. In the exemplary embodiment, datatransmission at the higher data rates are granted by the selected basestation 4 based on a set of system parameters such as the systemloading, fairness, and total throughput. An exemplary schedulingmechanism for high speed data transmission is described in detail in theaforementioned U.S. Pat. No. 6,335,922.

XV. Reverse Link Pilot/DRC Channel

The exemplary block diagram of the pilot/DRC channel is shown in FIG. 6.The DRC message is provided to DRC encoder 626, which encodes themessage in accordance with a predetermined coding format. Coding of theDRC message is important since the error probability of the DRC messageneeds to be sufficiently low because incorrect forward link data ratedetermination impacts the system throughput performance. In theexemplary embodiment, DRC encoder 626 is a rate (8,4) CRC block encoderthat encodes the 3-bit DRC message into an 8-bit code word. The encodedDRC message is provided to multiplier 628, which covers the message withthe Walsh code, which uniquely identifies the destination base station 4for which the DRC message is directed. The Walsh code is provided byWalsh generator 624. The covered DRC message is provided to multiplexer(MUX) 630, which multiplexes the message with the pilot data. The DRCmessage and the pilot data are provided to multipliers 650 a and 650 c,which spread the data with the PN_I and PN_Q signals, respectively.Thus, the pilot and DRC message are transmitted on both the inphase andquadrature phase of the sinusoid.

In the exemplary embodiment, the DRC message is transmitted to theselected base station 4. This is achieved by covering the DRC messagewith the Walsh code, which identifies the selected base station 4. Inthe exemplary embodiment, the Walsh code is 128 chips in length. Thederivation of 128-chip Walsh codes are known in the art. One uniqueWalsh code is assigned to each base station 4, which is in communicationwith mobile station 6. Each base station 4 decovers the signal on theDRC channel with its assigned Walsh code. The selected base station 4 isable to decover the DRC message and transmits data to the requestingmobile station 6 on the forward link in response thereto. Other basestations 4 are able to determine that the requested data rate is notdirected to them because these base stations 4 are assigned differentWalsh codes.

In the exemplary embodiment, the reverse link short PN codes for allbase stations 4 in the data communication system is the same and thereis no offset in the short PN sequences to distinguish different basestations 4. The data communication system of the present inventionsupports soft handoff on the reverse link. Using the same short PN codeswith no offset allows multiple base stations 4 to receive the samereverse link transmission from mobile station 6 during a soft handoff.The short PN codes provide spectral spreading but do not allow foridentification of base stations 4.

In the exemplary embodiment, the DRC message carries the requested datarate by mobile station 6. In the alternative embodiment, the DRC messagecarries an indication of the forward link quality (e.g., the C/Iinformation as measured by mobile station 6). Mobile station 6 cansimultaneously receive the forward link pilot signals from one or morebase stations 4 and performs the C/I measurement on each received pilotsignal. Mobile station 6 then selects the best base station 4 based on aset of parameters, which can comprise present and previous C/Imeasurements. The rate control information is formatted into the DRCmessage which can be conveyed to base station 4 in one of severalembodiments.

In the first embodiment, mobile station 6 transmits a DRC message basedon the requested data rate. The requested data rate is the highestsupported data rate which yields satisfactory performance at the C/Imeasured by mobile station 6. From the C/I measurement, mobile station 6first calculates the maximum data rate, which yields satisfactoryperformance. The maximum data rate is then quantized to one of thesupported data rates and designated as the requested data rate. The datarate index corresponding to the requested data rate is transmitted tothe selected base station 4. An exemplary set of supported data ratesand the corresponding data rate indices are shown in Table 1.

In the second embodiment, wherein mobile station 6 transmits anindication of the forward link quality to the selected base station 4,mobile station 6 transmits a C/I index, which represents the quantizedvalue of the C/I measurement. The C/I measurement can be mapped to atable and associated with a C/I index. Using more bits to represent theC/I index allows a finer quantization of the C/I measurement. Also, themapping can be linear or predistorted. For a linear mapping, eachincrement in the C/I index represents a corresponding increase in theC/I measurement. For example, each step in the C/I index can represent a2.0 dB increase in the C/I measurement. For a predistorted mapping, eachincrement in the C/I index can represent a different increase in the C/Imeasurement. As an example, a predistorted mapping can be used toquantize the C/I measurement to match the cumulative distributionfunction (CDF) curve of the C/I distribution as shown in FIG. 10.

Other embodiments to convey the rate control information from mobilestation 6 to base station 4 can be contemplated and are within the scopeof the present invention. Furthermore, the use of different number ofbits to represent the rate control information is also within the scopeof the present invention. Throughout much of the specification, thepresent invention is described in the context of the first embodiment,the use of a DRC message to convey the requested data rate, forsimplicity.

In the exemplary embodiment, the C/I measurement can be performed on theforward link pilot signal in the manner similar to that used in the CDMAsystem. A method and apparatus for performing the C/I measurement isdisclosed in U.S. patent application Ser. No. 08/722,763, entitled“METHOD AND APPARATUS FOR MEASURING LINK QUALITY IN A SPREAD SPECTRUMCOMMUNICATION SYSTEM,” filed Sep. 27, 1996, now U.S. Pat. No. 5,903,554,issued May 11, 1999, by Keith W. Saints, assigned to the assignee of thepresent invention and incorporated by reference herein. In summary, theC/I measurement on the pilot signal can be obtained by despreading thereceived signal with the short PN codes. The C/I measurement on thepilot signal can contain inaccuracies if the channel condition changedbetween the time of the C/I measurement and the time of actual datatransmission. In the present invention, the use of the FAC bit allowsmobile stations 6 to take into consideration the forward link activitywhen determining the requested data rate.

In the alternative embodiment, the C/I measurement can be performed onthe forward link traffic channel. The traffic channel signal is firstdespread with the long PN code and the short PN codes and decovered withthe Walsh code. The C/I measurement on the signals on the data channelscan be more accurate because a larger percentage of the transmittedpower is allocated for data transmission. Other methods to measure theC/I of the received forward link signal by mobile station 6 can also becontemplated and are within the scope of the present invention.

In the exemplary embodiment, the DRC message is transmits in the firsthalf of the time slot (see FIG. 7A). For an exemplary time slot of 1.667msec, the DRC message comprises the first 1024 chips or 0.83 msec of thetime slot. The remaining 1024 chips of time are used by base station 4to demodulate and decode the message. Transmission of the DRC message inthe earlier portion of the time slot allows base station 4 to decode theDRC message within the same time slot and possibly transmit data at therequested data rate at the immediate successive time slot. The shortprocessing delay allows the communication system of the presentinvention to quickly adapt to changes in the operating environment.

In the alternative embodiment, the requested data rate is conveyed tobase station 4 by the use of an absolute reference and a relativereference. In this embodiment, the absolute reference comprising therequested data rate is transmitted periodically. The absolute referenceallows base station 4 to determine the exact data rate requested bymobile station 6. For each time slots between transmissions of theabsolute references, mobile station 6 transmits a relative reference tobase station 4 which indicates whether the requested data rate for theupcoming time slot is higher, lower, or the same as the requested datarate for the previous time slot. Periodically, mobile station 6transmits an absolute reference. Periodic transmission of the data rateindex allows the requested data rate to be set to a known state andensures that erroneous receptions of relative references do notaccumulate. The use of absolute references and relative references canreduce the transmission rate of the DRC messages to base station 6.Other protocols to transmit the requested data rate can also becontemplated and are within the scope of the present invention.

XVI. Reverse Link Access Channel

The access channel is used by mobile station 6 to transmit messages tobase station 4 during the registration phase. In the exemplaryembodiment, the access channel is implemented using a slotted structurewith each slot being accessed at random by mobile station 6. In theexemplary embodiment, the access channel is time multiplexed with theDRC channel.

In the exemplary embodiment, the access channel transmits messages inaccess channel capsules. In the exemplary embodiment, the access channelframe format is identical to that specified by the IS-95 standard,except that the timing is in 26.67 msec frames instead of the 20 msecframes specified by IS-95 standard. The diagram of an exemplary accesschannel capsule is shown in FIG. 7B. In the exemplary embodiment, eachaccess channel capsule 712 comprises preamble 722, one or more messagecapsules 724, and padding bits 726. Each message capsule 724 comprisesmessage length (MSG LEN) field 732, message body 734, and CRC paritybits 736.

XVII. Reverse Link NACK Channel

In the present invention, mobile station 6 transmits the NACK messageson the data channel. The NACK message is generated for each packetreceived in error by mobile station 6. In the exemplary embodiment, theNACK messages can be transmitted using the Blank and Burst signalingdata format as disclosed in the aforementioned U.S. Pat. No. 5,504,773.

Although the present invention has been described in the context of aNACK protocol, the use of an ACK protocol can be contemplated and arewithin the scope of the present invention.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

1. A method in a communication system, comprising: measuring channelquality of multiple links in the communication system; selecting a linkof the multiple links based on the channel quality; periodicallytransmitting a quality indicator, wherein the quality indicator maps toa transmission data rate; and receiving data as a function of thequality indicator.
 2. The method as in claim 1, wherein periodicallytransmitting a quality indicator comprises: identifying a transmitterassociated with the selected link.
 3. The method as in claim 2, whereinperiodically transmitting a quality indicator comprises: covering thequality indicator to identify the transmitter associated with theselected link.
 4. The method as in claim 1, wherein a modulation formatcorresponds to the transmission data rate.
 5. The method as in claim 4,wherein the quality indicator is an index, wherein the index specifies amodulation format and a transmission data rate.
 6. The method as inclaim 5, further comprising: determining a transmission data rate basedon the channel quality; determining a modulation format as a function ofthe transmission data rate; determining a quality indicator as afunction of the transmission data rate and the modulation format; andcovering the quality indicator with a cover code to identify theselected link.
 7. The method as in claim 6, wherein the cover code is aWalsh code.
 8. The method as in claim 6, wherein periodicallytransmitting the quality indicator comprises: transmitting the qualityindicator to a transmitter associated with the selected link.
 9. Themethod as in claim 6, wherein selecting a link of the multiple linkscomprises: periodically selecting a link of the multiple links.
 10. Themethod as in claim 9, wherein the period for selecting the link is equalto the period for transmitting a quality indicator.
 11. The method as inclaim 1, wherein the selected link is a link having the highestCarrier-to-Interference (C/I).
 12. The method as in claim 1, wherein thequality indicator maps to a packet format.
 13. The method as in claim 1,wherein measuring channel quality comprises: measuringCarrier-to-Interference (C/I) of a reference signal for data ratecontrol.
 14. The method as in claim 13, wherein measuring channelquality further comprises: quantizing the Carrier-to-Interference (C/I)measurement.
 15. The method as in claim 14, wherein measuring channelquality further comprises: mapping the Carrier-to-Interference (C/I)measurement to a quality indicator.
 16. The method as in claim 1,wherein the selected link is maintained for a predetermined time period.17. The method as in claim 16, wherein the selected link is maintainedfor a predetermined number of time slots.
 18. The method as in claim 1,wherein the selected link is changed if another link has a channelquality which exceeds the channel quality of the selected link by ahysteresis value.
 19. An apparatus for a wireless communication system,comprising: means for measuring channel quality of multiple links in thecommunication system; means for selecting a link of the multiple linksbased on the channel quality; means for periodically transmitting aquality indicator, wherein the quality indicator maps to a transmissiondata rate; and means for receiving data as a function of the qualityindicator.
 20. The apparatus as in claim 19, wherein the means forperiodically transmitting a quality indicator comprises: means foridentifying a transmitter associated with the selected link.
 21. Theapparatus as in claim 20, wherein the means for periodicallytransmitting a quality indicator comprises: means for covering thequality indicator to identify the transmitter associated with theselected link.
 22. The apparatus as in claim 19, wherein a modulationformat corresponds to the transmission data rate.
 23. The apparatus asin claim 22, wherein the quality indicator is an index, wherein theindex specifies a modulation format and a transmission data rate. 24.The apparatus as in claim 23, further comprising: means for determininga transmission data rate based on the channel quality; means fordetermining a modulation format as a function of the transmission datarate; means for determining a quality indicator as a function of thetransmission data rate and modulation format; and means for covering thequality indicator with a cover code to identify the selected link. 25.The apparatus as in claim 24, wherein the cover code is a Walsh code.26. The apparatus as in claim 24, wherein the means for periodicallytransmitting the quality indicator comprises: means for transmitting thequality indicator to a transmitter associated with the selected link.27. The apparatus as in claim 24, wherein the means for selecting a linkof the multiple links comprises: means for periodically selecting a linkof the multiple links.
 28. The apparatus as in claim 27, wherein theperiod for selecting the link is equal to the period for transmitting aquality indicator.
 29. The apparatus as in claim 19, wherein theselected link is the link having the highest Carrier-to-Interference(C/I).
 30. The apparatus as in claim 19, wherein the quality indicatormaps to a packet format.
 31. The apparatus as in claim 19, wherein themeans for measuring channel quality comprises: means for measuringCarrier-to-Interference (C/I) of a reference signal for data ratecontrol.
 32. The apparatus as in claim 31, wherein the means formeasuring channel quality further comprises: means for quantizing theCarrier-to-Interference (C/l) measurement.
 33. The apparatus as in claim32, wherein the means for measuring channel quality further comprises:means for mapping the Carrier-to-Interference (C/I) measurement to aquality indicator.
 34. The apparatus as in claim 19, wherein theselected link is maintained for a predetermined number of time slots.35. The apparatus as in claim 19, wherein the selected link is changedif another link has a channel quality which exceeds the channel qualityof the selected link by a hysteresis value.
 36. The apparatus of any ofclaims 19 to 35, wherein the apparatus is a computer program product toincrease data throughput and efficiency when coupled to a computingdevice.
 37. A computer program product for a communication system,comprising: first plurality of instructions for measuring channelquality of multiple links in the communication system; second pluralityof instructions for selecting a link of the multiple links based on thechannel quality; third plurality of instructions for periodicallytransmitting a quality indicator, wherein the quality indicator maps toa transmission data rate; and fourth plurality of instructions forreceiving data as a function of the quality indicator.